There is continual market pressure to design portable devices to be smaller, smarter and more complex. In some portable devices, an audio device with multiple input signal sources that drives a speaker for both earpiece and hands free operation may be integrated into a single, mixed-signal sub-system. Such integration provides a better form factor and fewer external components.
Power consumption is one major issue faced by manufacturers of portable devices. This is one reason that manufacturers are using highly efficient class D power amplifiers in more and more handheld and other portable devices. However, a class D power amplifier may have a higher noise level than a class AB power amplifier, which can make the class D amplifier less suitable for earpiece or headphone applications.
One way to reduce printed circuit board space and component costs is to use the same speaker for both hands free and earpiece operation. One way to achieve this is to couple the single speaker to both the output of a class AB power amplifier (for earpiece operation) and the output of a class D power amplifier (for hands free operation).